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><A
NAME="INDEXES-ORDERING"
>11.4. Indexes and <TT
CLASS="LITERAL"
>ORDER BY</TT
></A
></H1
><P
>   In addition to simply finding the rows to be returned by a query,
   an index may be able to deliver them in a specific sorted order.
   This allows a query's <TT
CLASS="LITERAL"
>ORDER BY</TT
> specification to be honored
   without a separate sorting step.  Of the index types currently
   supported by <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>, only B-tree
   can produce sorted output &mdash; the other index types return
   matching rows in an unspecified, implementation-dependent order.
  </P
><P
>   The planner will consider satisfying an <TT
CLASS="LITERAL"
>ORDER BY</TT
> specification
   either by scanning an available index that matches the specification,
   or by scanning the table in physical order and doing an explicit
   sort.  For a query that requires scanning a large fraction of the
   table, an explicit sort is likely to be faster than using an index
   because it requires
   less disk I/O due to following a sequential access pattern.  Indexes are
   more useful when only a few rows need be fetched.  An important
   special case is <TT
CLASS="LITERAL"
>ORDER BY</TT
> in combination with
   <TT
CLASS="LITERAL"
>LIMIT</TT
> <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>: an explicit sort will have to process
   all the data to identify the first <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> rows, but if there is
   an index matching the <TT
CLASS="LITERAL"
>ORDER BY</TT
>, the first <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>
   rows can be retrieved directly, without scanning the remainder at all.
  </P
><P
>   By default, B-tree indexes store their entries in ascending order
   with nulls last.  This means that a forward scan of an index on
   column <TT
CLASS="LITERAL"
>x</TT
> produces output satisfying <TT
CLASS="LITERAL"
>ORDER BY x</TT
>
   (or more verbosely, <TT
CLASS="LITERAL"
>ORDER BY x ASC NULLS LAST</TT
>).  The
   index can also be scanned backward, producing output satisfying
   <TT
CLASS="LITERAL"
>ORDER BY x DESC</TT
>
   (or more verbosely, <TT
CLASS="LITERAL"
>ORDER BY x DESC NULLS FIRST</TT
>, since
   <TT
CLASS="LITERAL"
>NULLS FIRST</TT
> is the default for <TT
CLASS="LITERAL"
>ORDER BY DESC</TT
>).
  </P
><P
>   You can adjust the ordering of a B-tree index by including the
   options <TT
CLASS="LITERAL"
>ASC</TT
>, <TT
CLASS="LITERAL"
>DESC</TT
>, <TT
CLASS="LITERAL"
>NULLS FIRST</TT
>,
   and/or <TT
CLASS="LITERAL"
>NULLS LAST</TT
> when creating the index; for example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX test2_info_nulls_low ON test2 (info NULLS FIRST);
CREATE INDEX test3_desc_index ON test3 (id DESC NULLS LAST);</PRE
><P>
   An index stored in ascending order with nulls first can satisfy
   either <TT
CLASS="LITERAL"
>ORDER BY x ASC NULLS FIRST</TT
> or
   <TT
CLASS="LITERAL"
>ORDER BY x DESC NULLS LAST</TT
> depending on which direction
   it is scanned in.
  </P
><P
>   You might wonder why bother providing all four options, when two
   options together with the possibility of backward scan would cover
   all the variants of <TT
CLASS="LITERAL"
>ORDER BY</TT
>.  In single-column indexes
   the options are indeed redundant, but in multicolumn indexes they can be
   useful.  Consider a two-column index on <TT
CLASS="LITERAL"
>(x, y)</TT
>: this can
   satisfy <TT
CLASS="LITERAL"
>ORDER BY x, y</TT
> if we scan forward, or
   <TT
CLASS="LITERAL"
>ORDER BY x DESC, y DESC</TT
> if we scan backward.
   But it might be that the application frequently needs to use
   <TT
CLASS="LITERAL"
>ORDER BY x ASC, y DESC</TT
>.  There is no way to get that
   ordering from a plain index, but it is possible if the index is defined
   as <TT
CLASS="LITERAL"
>(x ASC, y DESC)</TT
> or <TT
CLASS="LITERAL"
>(x DESC, y ASC)</TT
>.
  </P
><P
>   Obviously, indexes with non-default sort orderings are a fairly
   specialized feature, but sometimes they can produce tremendous
   speedups for certain queries.  Whether it's worth maintaining such an
   index depends on how often you use queries that require a special
   sort ordering.
  </P
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